Wide and Deep Oxide Trench in A Semiconductor Substrate with Interspersed Vertical Oxide Ribs

ABSTRACT

A semiconductor device structure with an oxide-filled large deep trench (OFLDT) portion having trench size TCS and trench depth TCD is disclosed. A bulk semiconductor layer (BSL) is provided with a thickness BSLT&gt;TCD. A large trench top area (LTTA) is mapped out atop BSL with its geometry equal to OFLDT. The LTTA is partitioned into interspersed, complementary interim areas ITA-A and ITA-B. Numerous interim vertical trenches of depth TCD are created into the top BSL surface by removing bulk semiconductor materials corresponding to ITA-B. The remaining bulk semiconductor materials corresponding to ITA-A are converted into oxide. If any residual space is still left between the so-converted ITA-A, the residual space is filled up with oxide deposition. Importantly, the geometry of all ITA-A and ITA-B should be configured simple and small enough to facilitate fast and efficient processes of oxide conversion and oxide filling.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of a pending U.S. patentapplication entitled “Method of Filling Large Deep Trench with HighQuality Oxide for Semiconductor Devices” application Ser. No.12/637,988, filing date: Dec. 15, 2009, inventor Xiaobin Wang (attorneydocket# APOM032). The above contents are incorporated herein byreference for any and all purposes.

FIELD OF INVENTION

This invention relates generally to the field of power semiconductordevice structure and fabrication. More specifically, the presentinvention is directed to fabrication method for large deep oxide trenchof a power semiconductor device structure.

BACKGROUND OF THE INVENTION

Power semiconductor devices have many industrial applications, such aspower amplifiers, power convertors, low noise amplifiers and digitalIntegrated Circuits (IC) to name a few. Some examples of powersemiconductor devices are Schottky diode, Metal-Oxide-SemiconductorField Effect Transistor (MOSFET), Insulated Gate Bipolar Transistor(IGBT) and double diffused Metal-Oxide-Semiconductor Transistor (DMOS).The termination structure of power semiconductor devices often requiresa high quality semiconductor oxide layer such as silicon oxide. Formedium to high voltage devices, a high quality semiconductor oxide layerthat is both deep and wide (for example, of the order of ten microns) isoften required to insure a high breakdown voltage V_(bk) and low leakagecurrent I_(lk). While semiconductor oxide layers of thickness around 1micron can be thermally formed or deposited, it can take more than twohours process time already just to form a 0.5 micron thick thermaloxide. Besides being of lower quality, a deposited oxide thickness of afew microns is already considered quite thick in that its dielectricproperty non-uniformity can be a problem. Manufacturing issues withforming a deep and wide oxide filled trench include: processing time,non-uniformity, and high stress levels.

FIG. 4 illustrates a prior art termination structure disclosed in U.S.Pat. No. 6,309,929 entitled “METHOD OF FORMING TRENCH MOS DEVICE ANDTERMINATION STRUCTURE” by Hsu et al and granted on Oct. 30, 2001. Thesimultaneously fabricated trench MOS device, located to the left of thetermination structure, is not shown here to avoid unnecessary obscuringdetails. The termination structure includes a number of Schottky diodesformed between an anode electrode 160A (delimited by a photoresistpattern 165) and first semiconductor layer 100A at numerous mesa surfacelocations 115A. The Schottky diodes are, through the anode electrode160A, electrically connected to a number of MOS structures made of firstconductive layer 140, gate oxide layer 125 and the first semiconductorlayer 100A. The MOS structures include a poly spacer 122 bordering animportant dielectric layer 150 for sustaining a high voltage dropbetween second substrate 100B and the anode electrode 160A. In anembodiment, the dielectric layer 150 is a termination oxide directlyformed upon a trench. Either thermally grown or deposited, the so formedtermination oxide thickness is limited to only about 0.2 micron to aboutseveral microns due to manufacturing equipment tooling and silicon-oxideinterface stress limitations. So the formation of deep and wide highquality semiconductor oxide layer under this prior art can be an issue.

In a paper entitled “Feasibility study of a junction termination usingdeep trench isolation technique for the realization of DT-SJMOSFETs”published in Proceedings of the 20^(th) International Symposium on PowerSemiconductor Devices & IC's, May 18-22, 2008, Orlando, Fla. byMahfoz-Kotb et al, an alternative dielectric material calledBenzoCycloButene (BCB) was tried in lieu of semiconductor oxide for deeptrench fill. Besides complicating the associated device fabricationprocess with an added new material (BCB), the resulting dielectricmaterial quality was found to be inferior compared to thermal oxide.Thus, there is an ongoing need of fabricating high quality trenchsemiconductor oxide layer that is both deep and wide without incurring ahigh thermal budget and with good production throughput.

SUMMARY OF THE INVENTION

A method is disclosed for creating a semiconductor device structure withan oxide-filled large deep trench (OFLDT) portion having trench size TCSand trench depth TCD. The method includes:

-   -   a) Provide a bulk semiconductor layer (BSL) with a thickness        BSLT>TCD. Map out a large trench top area (LTTA) atop the BSL        with its geometry equal to that of OFLDT.    -   b) Partition the LTTA into interspersed, complementary interim        areas ITA-A and ITA-B each of pre-determined geometry.    -   c) Create a number of interim vertical trenches till the depth        TCD into the top BSL surface so as to remove bulk semiconductor        materials corresponding to ITA-B.    -   d) Convert, via thermal oxidation, the remaining bulk        semiconductor materials corresponding to ITA-A into oxide.        Afterwards, if there is still any residual space left between        the so-converted ITA-A, fill up the residual space with oxide        deposition.    -   e) Planarize, via chemical mechanical polishing (CMP), the top        surface as desired for any additional processing.

In a more specific important embodiment, the geometry of all partitionedITA-A and ITA-B should be configured to be simple and small enough so asto facilitate:

-   -   d1) A fast and efficient process of converting the remaining        bulk semiconductor materials corresponding to ITA-A into high        quality oxide.    -   d2) A fast and efficient process of filling the residual space        with high quality oxide.

In a related embodiment of application involving coexisting activetrench devices, the step of creating a number of interim verticaltrenches can be expanded to simultaneously create a number of deepactive device trenches in an active device trench top area ADTTA mappedout atop the BSL and separate from the LTTA. As a result, the step ofconverting the remaining bulk semiconductor materials corresponding toITA-A into oxide also simultaneously converts a surface portion of thedeep active device trench walls into oxide. Next, the top surface isplanarized and the converted oxide in the ADTTA can be selectively andpartially etched down, through a correspondingly windowed mask, to leavea liner oxide upon the deep active device trenches.

As an alternative to the above after simultaneous creation of theinterim vertical trenches and the deep active device trenches:

-   -   An interim oxidation prevention layer, for example silicon        nitride, can be selectively formed upon the deep active device        trenches.    -   The remaining bulk semiconductor materials corresponding to        ITA-A can then be selectively converted into oxide. Afterwards,        if there is still any residual space left between the        so-converted ITA-A, fill up the residual space with oxide        deposition.    -    Finally, the interim oxidation prevention layer is removed and        a liner oxide can be formed upon the now exposed deep active        device trenches.

As a refinement, the step of creating the deep active device trenchesand the interim vertical trenches includes:

-   -   1. Anisotropically etch out most volumetric portion of the deep        active and interim vertical trenches.    -   2. Isotropically etch out small remaining volumetric portion of        the deep active and interim vertical trenches thus causing them        to have rounded corners to facilitate subsequent processing.

In a more specific embodiment, the shape of each ITA-A can be made intoa square, rectangle, circle, ellipse, hexagon or polygon. The size ofeach ITA-A is from about 0.5 micron to about 5 microns and the size ofeach ITA-B is from about 0.4 micron to about 3 microns. However it maybe desirable for the cell pitch of the interim trenches (ITA-A plusITA-B) to be similar to the cell pitch of the active area device cells.(e.g., within +/−75%). TCS is from about 10 micron to about 100 micronand TCD is from about 10 micron to about 50 micron. However there isreally no limit to how wide TCS can be, and TCD may certainly be madeless than 10 microns should the application call for it. In order toeasily fill the remaining gaps after thermal oxidation, ITA-A and ITA-Bshould be sized such that after thermal oxidation, the remaining gap inthe interim trenches is less than 4 microns.

In essence, this invention shows how to form a wide and deep oxidetrench comprising vertical ribs made of thermal oxide interspersedthroughout the wide oxide trench. Any remaining gaps in the wide oxidetrench between the oxide ribs may be filled with insulating materialsuch as deposited oxide. The wide and deep oxide trench can function asa termination trench for a semiconductor device. The oxide ribs maycontain a small notch at their bottoms.

These aspects of the present invention and their numerous embodimentsare further made apparent, in the remainder of the present description,to those of ordinary skill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully describe numerous embodiments of the presentinvention, reference is made to the accompanying drawings. However,these drawings are not to be considered limitations in the scope of theinvention, but are merely illustrative:

FIG. 4 illustrates a prior art power semiconductor device terminationstructure with an integrated dielectric layer for sustaining a highvoltage drop across it;

FIG. 1A, FIG. 1A-1, FIG. 1A-2, FIG. 1B, FIG. 1B-1 and FIG. 1B-2 are topviews and some associated process steps of two bulk semiconductor layersillustrating a present invention idea of creating interspersedsemiconductor pillars and interim vertical trenches within a largetrench top area for large deep trench oxide;

FIG. 2A-0, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D and FIG. 2E illustrate afirst embodiment method under the present invention of simultaneouslycreating a semiconductor device structure with an oxide-filled largedeep trench portion and another portion of deep active device trenches;and

FIG. 3A through FIG. 3D illustrate a second embodiment method under thepresent invention of simultaneously creating a semiconductor devicestructure with an oxide-filled large deep trench portion and anotherportion of deep active device trenches.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The description above and below plus the drawings contained hereinmerely focus on one or more currently preferred embodiments of thepresent invention and also describe some exemplary optional featuresand/or alternative embodiments. The description and drawings arepresented for the purpose of illustration and, as such, are notlimitations of the present invention. Thus, those of ordinary skill inthe art would readily recognize variations, modifications, andalternatives. Such variations, modifications and alternatives should beunderstood to be also within the scope of the present invention.

FIG. 1A, FIG. 1A-1, FIG. 1A-2, FIG. 1B, FIG. 1B-1 and FIG. 1B-2 are topviews and some associated process steps of two bulk semiconductor layersillustrating a present invention idea of creating interspersedsemiconductor pillars and interim vertical trenches within a largetrench top area for large deep trench oxide. FIG. 1A and FIG. 1B are topviews of two bulk semiconductor layers (BSL) 1 a and BSL 1 b. While notvisible in these views, both BSL 1 a and BSL 1 b have a bulksemiconductor layer thickness (BSLT) along the Z-axis. The top surfaceof BSL 1 a in FIG. 1A is mapped into a large trench top area (LTTA) 2 afor an oxide-filled large deep trench (OFLDT) of trench size TCS andtrench depth TCD and an active device trench top area ADTTA 3 a foractive device trenches. The top surface geometry of LTTA 2 a is maderoughly equal to that of OFLDT. The LTTA is slightly smaller than thearea of the OFLDT to take into account the thermal oxidation step laterin the process. Selected portions of the bulk semiconductor materialswithin BSL 1 a are anisotropically removed to create an interim verticaltrench 50 a interspersed with a number of remaining semiconductorpillars 10 a, 11 a, 12 a, . . . , 21 a. While also invisible, theinterim vertical trench 50 a is made with a trench depth TCD<BSLT.Selected portions of the bulk semiconductor materials within the activedevice trench top area ADTTA 3 a may also be anisotropically removed tocreate a desired number of deep active device trenches 40 a, 41 aseparated by active device semiconductor mesas 25 a, 26 a asillustrated. In practice, the creation of interim vertical trench 50 aand deep active device trenches 40 a, 41 a can be carried out inseparate steps for individually selectable trench depths or carried outsimultaneously for the same trench depth and lower cost. As shown inFIG. 1A-1, the remaining bulk semiconductor materials of semiconductorpillars 10 a, 11 a, 12 a, . . . , 21 a are then converted intosemiconductor oxide, e.g., by thermal oxidation, to form oxide pillars22. This oxidation process also converts the exposed sidewalls of thetrenches 40 a, 41 a, and 50 a to sidewall oxides 42, 51. Afterwards, ifthere is still any residual space left between the so-convertedsemiconductor oxide pillars, the residual space can be filled up withoxide deposition thus completing the OFLDT 52 as shown in FIG. 1A-2.

The top surface of BSL 1 b in FIG. 1B is mapped into an LTTA 2 b for anoxide-filled large deep trench (OFLDT) of trench size TCS and trenchdepth TCD and an ADTTA 3 b for active device trenches. The top surfacegeometry of LTTA 2 b is made equal to that of OFLDT. Selected portionsof the bulk semiconductor materials within BSL 1 b are anisotropicallyremoved to create interim vertical trenches 50 b, 51 b, 52 b, 53 binterspersed with a number of remaining semiconductor mesas 10 b, 11 b,12 b. In a separate area, selected portions of the bulk semiconductormaterials within the ADTTA 3 b are also anisotropically removed tocreate a desired number of deep active device trenches 40 b, 41 bseparated by active device semiconductor mesas 15 b, 16 b asillustrated. In practice, the creation of interim vertical trenches 50b, 51 b, 52 b, 53 b and deep active device trenches 40 b, 41 b can becarried out in separate steps for individually selectable trench depthsor carried out simultaneously for the same trench depth. The remainingbulk semiconductor materials of semiconductor mesas 10 b, 11 b, 12 b arethen converted into semiconductor oxide, e.g., by thermal oxidation, toform converted oxides 70 b, 71 b, 72 b. This oxidation process alsoconverts the exposed sidewalls of the trenches 40 b, 41 b, and 50 b, 51b, 52 b, 53 b to sidewall oxides 77, 78. Afterwards, if there is stillany residual space left between the so-converted semiconductor oxidemesas, the residual space can be filled up with oxide deposition thuscompleting the OFLDT 79, as shown in FIG. 1B-2. By way of example, theoxidation deposition may be planarized back to the semiconductorsurface. These process steps will be explained more thoroughly below.

FIG. 2A-0, FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D and FIG. 2E illustrate,via side cross sectional views, a detailed first embodiment method underthe present invention of simultaneously creating a semiconductor devicestructure with an oxide-filled large deep trench portion and anotherportion of deep active device trenches. In FIG. 2A-0, a bulksemiconductor layers (BSL) 1 b of thickness BSLT is provided. In FIG.2A, with the previously mentioned interspersed, complementary interimareas ITA-A and ITA-B shown as labeled, the BSL 1 b is mapped into aLTTA 2 b atop the BSL with its geometry and size TCS roughly equal tothat of OFLDT. The top surface of BSL 1 b is then anisotropically etchedto a depth TCD<BSLT through a windowed mask to create, within the LTTA 2b, interim vertical trenches 50 b, 51 b, 52 b, 53 b interspersed with anumber of remaining semiconductor mesas 10 b, 11 b, 12 b. These figuresare not to scale, as the BSLT is usually many times thicker than theTCD. The ADTTA 3 b is also anisotropically etched through a windowedmask to create a desired number of deep active device trenches 40 b, 41b separated by active device semiconductor mesas 15 b, 16 b. Notice theappearance of rounded corners 60 at the bottom of the interim verticaltrenches and the deep active device trenches. As will be presentlyexplained, these rounded corners 60 serve to facilitate corner coverageof subsequent processing steps. The rounded corners 60 may also helpimproved breakdown voltages, by reducing electric field crowding due tosharp curvatures. The rounded corners 60 can be created with thefollowing steps:

-   -   1. Anisotropically etch out most volumetric portion of the        interim vertical and deep active trenches.    -   2. Isotropically etch out small remaining volumetric portion of        the interim vertical and deep active trenches thus causing them        to have rounded corners.

FIG. 2B illustrates the completed conversion of bulk semiconductormaterials of the remaining semiconductor mesas 10 b, 11 b, 12 b andtrench sidewalls respectively into converted oxides 70 b, 71 b, 72 b and68 b and 69 b separated by residual spaces 90 b, 91 b, 92 b, 93 b. Theconversion can be carried out through thermal oxidation resulting in,for example, a silicon dioxide layer thickness from 2500 Angstrom ˜5000Angstrom. Notice that due to substantial difference of molecularvolumetric density between the semiconductor material and its oxide, thesize of the converted oxides 70 b, 71 b, 72 b has “grown” to besubstantially bigger than their predecessor semiconductor mesas 10 b, 11b, 12 b. Notice that at the bottom of the converted oxides 70 b, 71 b,72 b there may be residual notches 95 where the oxides at the bottom ofthe trenches meet. Simultaneously, the same oxide conversion process hasalso converted the surface portion of the much larger sized (relative tothe semiconductor mesas 10 b-12 b) active device semiconductor mesas 15b, 16 b into converted oxides 75 b and 76 b separated by residual spaces80 b, 81 b. It is important to point out that, had the various interimvertical trenches 50 b, 51 b, 52 b, 53 b of FIG. 2A been alternativelymade sufficiently smaller (i.e., narrower along X-direction) thanindicated there, the growth of the converted oxides 70 b, 71 b, 72 bwould have completely joined their neighbors within the LTTA 2 b with noresidual space left thus completing a large deep oxide trench with depthTCD and size TCS already. FIG. 2C illustrates the completion of fillingup the residual spaces (90 b, 91 b, 92 b, 93 b) and (80 b, 81 b) withdeposited fill oxide 100 b up to a fill oxide surface 101 b thuscompleting a large deep oxide trench with depth TCD and size TCS aswell. The residual spaces (90 b, 91 b, 92 b, 93 b) are sized so thatthey can be easily filled with oxide or another suitable material. Theactive and termination regions are carefully designed, so that after thesemiconductor mesas 10 b, 11 b, 12 b are converted to converted oxides70 b, 71 b, 72 b, the active and termination region have the same orsimilar trench density, or cell pitch, in which case high quality HDP(High Density Plasma) oxide deposition can easily fill the residualspaces (90 b, 91 b, 92 b, 93 b). HDP oxide deposition cannot be used tofill a very wide gap due to tooling and stress limitations, which is whythis invention divides an eventual wide trench into small trenches. Inan alternative embodiment, a different insulating material may be usedin place of fill oxide 100 b.

Thus, by dividing an LTTA 2 b, designated for an OFLDT of trench sizeTCS and trench depth TCD, into numerous interspersed complementaryinterim semiconductor mesas 10 b-12 b and interim vertical trenches 50b-53 b with both mesa and trench sizes being much smaller than TCS, thepresent invention can indirectly create the OFLDT with complementaryconverted oxides and deposited oxides while avoiding quality problemsarising from an otherwise direct creation of the OFLDT. The crosssectional geometry of all interim semiconductor mesas and interimvertical trenches should be configured to be simple (to avoid deadcorners) and small enough so as to facilitate:

-   -   1. A fast and efficient oxide conversion process (FIG. 2B).    -   2. A fast and efficient oxide filling process (FIG. 2C).

As for some specific examples, the cross sectional shape of each interimsemiconductor mesa can be made into a square, rectangle, circle,ellipse, hexagon or polygon or any other suitable shape. The crosssectional size of each interim semiconductor mesa can be from about 0.5micron to about 4 microns and the cross sectional size of each interimvertical trench can be from about 0.4 micron to about 3 micron. Ideallythe cell pitch of the interim vertical trenches are similar to the cellpitch of the active cell devices, e.g. within +/−75%. This helps for theoxide deposition step to be more uniform which in turn aids theplanarization step. Correspondingly, the resulting TCS can be from about10 micron to about 100 micron and TCD can be from about several micronsto about 50 micron. Furthermore, the process throughput of the presentinvention can be made higher by partitioning the LTTA 2 b for a highersurface area-to-volume ratio of both the interim semiconductor mesas andthe interim vertical trenches. To those skilled in the art, by now itshould become clear that the aforementioned rounded corners 60 at thebottom of the interim vertical trenches and the deep active devicetrenches serve to facilitate corner coverage of both the oxideconversion step and the oxide filling step, and to improve the electricfield characteristics.

FIG. 2D illustrates the planarization, via chemical mechanical polishing(CMP) for example, of the fill oxide surface 101 b (FIG. 2C) down to aplanarized fill oxide surface 102 b to facilitate subsequent processing.In FIG. 2E, the converted oxides 75 b, 76 b in the ADTTA 3 b has beenselectively and partially etched down, through a correspondinglywindowed mask 110 b, to leave liner oxides 105 b, 106 b upon the deepactive device trenches 40 b, 41 b. Alternatively, all oxide in the deepactive device trenches 40 b, 41 b can be completely removed first thenthinner liner oxides can be re-grown therein if needed.

FIG. 3A through FIG. 3D illustrate, via side cross sectional views, adetailed second embodiment method under the present invention ofsimultaneously creating a semiconductor device structure with anoxide-filled large deep trench portion and another portion of deepactive device trenches. In FIG. 3A, with the previously mentionedinterspersed, complementary interim areas ITA-A and ITA-B also shown aslabeled, the top surface of BSL 1 b has been anisotropically etched to adepth TCD<BSLT through a windowed mask to create, within the LTTA 2 b,interim vertical trenches 50 b, 51 b, 52 b interspersed with a number ofremaining semiconductor mesas 10 b, 11 b. The ADTTA 3 b has also beenanisotropically etched through a windowed mask to create a deep activedevice trench 40 b bounded by an active device semiconductor mesa 16 b.Notice also the appearance of rounded corners 60 at the bottom of theinterim vertical trenches and the deep active device trenches.

FIG. 3B illustrates the selective formation of an interim oxidationprevention layer 120 b, for example silicon nitride, upon the deepactive device trench 40 b.

FIG. 3C illustrates, except for the bulk semiconductor materials beneaththe interim oxidation prevention layer 120 b within the deep activedevice trench 40 b, the oxidation of all the exposed silicon surfaces,including the completed conversion of bulk semiconductor materials ofthe remaining semiconductor mesas 10 b, 11 b and trench sidewalls intoconverted oxides 70 b, 71 b and 68 b and 69 b, separated by residualspaces 90 b, 91 b, 92 b. The conversion can be carried out through wetoxidation for example. Similar to FIG. 2B before, due to substantialdifference of molecular volumetric density between the semiconductormaterial and its oxide, the size of the converted oxides 70 b, 71 b has“grown” to be substantially bigger than their predecessor semiconductormesas 10 b, 11 b. Additionally, had the various interim verticaltrenches 50 b, 51 b, 52 b of FIG. 3A been alternatively madesufficiently smaller (i.e., narrower along X-direction) than indicatedthere, the growth of the converted oxides 70 b, 71 b, 72 b would havecompletely joined their neighbors within the LTTA 2 b with no residualspace left thus completing a large deep oxide trench with depth TCD andsize TCS already.

FIG. 3D illustrates:

-   -   1. The completion of filling up the residual spaces (90 b, 91 b,        92 b) with deposited fill oxide 100 b up to a fill oxide surface        101 b thus completing a large deep oxide trench with depth TCD        and size TCS.    -   2. The differential removal of the interim oxidation prevention        layer 120b.    -   3. The formation of liner oxide 106 b upon the now exposed deep        active device trench 40 b.

A method has been invented for making large deep trenches filled withhigh quality oxide for semiconductor devices. Throughout the descriptionand drawings, numerous exemplary embodiments were given with referenceto specific configurations. It will be appreciated by those of ordinaryskill in the art that the present invention can be embodied in numerousother specific forms and those of ordinary skill in the art would beable to practice such other embodiments without undue experimentation.The scope of the present invention, for the purpose of the presentpatent document, is hence not limited merely to the specific exemplaryembodiments of the foregoing description, but rather is indicated by thefollowing claims. Any and all modifications that come within the meaningand range of equivalents within the claims are intended to be consideredas being embraced within the spirit and scope of the present invention.

1. A method of forming a large deep oxide trench in a semiconductorsubstrate comprising: a) forming, in the semiconductor substrate, aplurality of interim trenches separated by a corresponding number ofremaining semiconductor mesas between the interim trenches; and b)oxidizing the remaining semiconductor mesas until they are substantiallyconverted to oxide.
 2. The method of claim 1 further comprising: c)filling up any remaining spaces of the interim trenches with insulatingmaterial.
 3. The method of claim 2 wherein filling up any remainingspaces further comprises filling up any remaining spaces of the interimtrenches with oxide.
 4. The method of claim 1 wherein oxidizing theremaining semiconductor mesas is via thermal oxidation.
 5. The method ofclaim 1 wherein oxidizing the remaining semiconductor mesas furthercomprises oxidizing the semiconductor mesas until all the semiconductormaterial in the mesas is converted to oxide.
 6. A wide and deep oxidetrench in a semiconductor substrate comprising vertical oxide ribs,being made of thermal oxide, interspersed throughout the wide and deepoxide trench.
 7. The wide and deep oxide trench of claim 6 wherein anyremaining spaces between the vertical oxide ribs are filled with aninsulating material.
 8. The wide and deep oxide trench of claim 6wherein any remaining spaces between the vertical oxide ribs are filledwith deposited oxide.
 9. The wide and deep oxide trench of claim 6wherein the vertical oxide ribs contain a small notch at their bottoms.